Method and computer and imaging apparatus for evaluating medical image data

ABSTRACT

In a method, an evaluation computer, and a medical imaging apparatus for evaluating medical images of an organ system of an examination subject, wherein the organ system has a first side and a second side that are characteristically bilaterally symmetrical to one another, first and second medical image datasets of the organ system of the examination subject are acquired and processed to obtain a result image dataset by a global image data subtraction in which image components of the first and second medical image data are subtracted from one another, and a symmetry subtraction in which image components of the first side and the second side of the organ system are subtracted from one another within a medical image dataset.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention concerns a method for evaluating medical image data, andan evaluation computer, a medical imaging apparatus, and anon-transitory, computer-readable data storage medium for implementingsuch a method.

Description of the Prior Art

Medical image data are normally acquired by medical imaging apparatusesand can represent anatomical structures and/or functional processes ofthe body of an examination subject. Medical image data of a singleexamination subject often are composed of medical image datasets thathave been recorded at different points in time, for example. A typicalproblem to be addressed in this regard is how to compare the multiplemedical image datasets with one another and identify variations betweenthe multiple medical image datasets. For example, a characteristic of achange in the body of the examination subject as a function of time canbe determined in a dynamic measurement.

In a perfusion imaging procedure, for example, there are typically anumber of chronologically sequential medical image datasets availablethat describe a change in the content of a contrast agent in a tissue ofthe examination subject. A metric for a blood flow through the tissuecan be derived therefrom.

In functional magnetic resonance imaging, signal variations between anumber of medical image datasets typically indicate changes in the localoxygen uptake rate. From this it is possible to derive a metric for achange in the activity of functional centers in the brain of theexamination subject, particularly in relation to a reaction to specificstimuli, such as optical stimuli.

Dynamic nuclear medical measurements, such as dynamic positron emissiontomography (PET) measurements, are also known. In this case it ispossible to measure the distribution of certain tracers, such asfluorodeoxyglucose, in the body of the examination subject over time.

Other medical imaging methods in which a number of medical imagedatasets of an examination subject are acquired, and are to be compared,are known to those skilled in the art.

Typically, it is difficult to detect minor variations between the numberof medical image datasets in a qualitative sense. Dynamic measurementsin particular are often limited in their signal-to-noise ratio due totheir temporal resolution, with the result that variations in themedical image datasets are frequently hidden in the image noise.

SUMMARY OF THE INVENTION

An object of the invention is to enable an improved evaluation ofmedical image data composed of a number of medical image datasets.

The method according to the invention for evaluating medical image data,which image an organ system of an examination subject wherein the organsystem has a first side and a second side that are bilaterallysymmetrical to one another with respect to anatomical characteristics,includes the following steps.

A first medical image dataset of the organ system of the examinationsubject is acquired and a second medical image dataset of the organsystem of the examination subject is also acquired. The first medicalimage dataset and the second medical image dataset are processed toobtain a result image dataset that is provided as a data file forviewing. The processing of the first medical image dataset and thesecond medical image dataset includes a global image data subtraction inwhich image components of the first medical image data and the secondmedical image data are subtracted from one another, and a symmetrysubtraction in which image components of the first side and second sideof the organ system are subtracted from one another within a medicalimage dataset.

The examination subject may be a patient, a training volunteer or aphantom. The organ system is characteristically bilaterally symmetricalwith respect to a plane of symmetry. Thus, the first side of the organsystem lies on a first side of the plane of symmetry and the second sideof the organ system lies on a second side of the plane of symmetry.Typically, the organ system is characteristically bilaterallysymmetrical with respect to a sagittal plane of symmetry. In a humanexamination subject, such a sagittal plane of symmetry typically extendsfrom the head to the pelvis of the examination subject and from thespine to the abdomen of the examination subject. Typically, the plane ofsymmetry runs through the center of the examination subject's body andcan therefore be referred to as the median plane.

The method according to the invention can be applied in an imaging ofthe brain of the examination subject, as a bilaterally symmetricalorgan. The first side of the organ system can then be the left cerebralhemisphere and the second side of the organ system can be the rightcerebral hemisphere. The reverse case is, of course, also possible.Other possible bilaterally symmetrical organs are, for example, a partof the skeletal system and/or muscular system of the examinationsubject, a pair of eyes of the examination subject, a pair of breasts ofthe examination subject, a pair of kidneys of the examination subject, apair of lungs of the examination subject, etc. Other bilaterallysymmetrical organs are also known to those skilled in the art. It shouldbe noted that a perfect bilateral symmetry is practically unheard of innature. Typically, therefore, the first side and the second side of theorgan system are characteristically substantially bilaterallysymmetrical to one another, meaning that deviations from perfectbilateral symmetry are diagnostically irrelevant. For example, the firstside and the second side of the organ system may be considered perfectlybilaterally symmetrical and/or mirror-symmetrical only after beingbrought into registration to one another, such as by a non-rigidregistration.

The acquisition of the first medical image dataset and/or the secondmedical image dataset can take place by operation of a medical imagingapparatus, particularly a scanner thereof. Alternatively, theacquisition of the first medical image dataset and/or the second medicalimage dataset can be the loading of an already-recorded first medicalimage dataset and/or an already recorded second medical image datasetfrom an image database.

The first medical image dataset and the second medical image dataset maprepresent the same organ system of the same examination subject. Thefirst medical image dataset and the second medical image dataset mayhave been recorded at different points in time by a medical imagingdevice. A relatively long period of time may elapse between therecording of the first medical image dataset and the recording of thesecond medical image dataset. Thus, the first medical image dataset andthe second medical image dataset may have been recorded in differentmeasurements, on different days, for example. Alternatively, the firstmedical image dataset and the second medical image dataset may have beenrecorded in a single, dynamic, measurement. A relevant event may occurbetween the recording of the first medical image dataset and the secondmedical image dataset. Such a relevant event may be, for example, astimulus, such as an optical or acoustic stimulus, to the examinationsubject, an administration of contrast agent or some other intervention.In this case the first medical image dataset may have been recorded at apoint in time prior to the second medical image dataset or vice versa.

The processing of the first medical image dataset and the second medicalimage dataset is implemented by an algorithm in a computer. In this casethe algorithm receives the first medical image dataset and the secondmedical image dataset as input parameters. The algorithm generates theresult image dataset as an output. The provision of the result imagedataset can be displaying the result image dataset for a user on adisplay monitor and/or storing the result image dataset in a database.The result image dataset includes information that represents a changein the contents of an image between the first medical image dataset andthe second medical image dataset.

In accordance with the invention, the processing of the first medicalimage dataset and the second medical image dataset includes twosubtractions, i.e. the global image data subtraction and the symmetrysubtraction. The processing may also involve further processing steps,for example further subtractions. Preferably, the global image datasubtraction is performed first, followed by the symmetry subtraction. Itis also possible, however, for the symmetry subtraction to be performedfirst, and then the global image data subtraction.

Image components of both the first medical image dataset and the secondmedical image dataset are used for the global image data subtraction.The global image data subtraction therefore constitutes a subtractionwith respect to time. By the global image data subtraction, it istherefore possible to identify changes over time between the recordingof the first medical image dataset and the recording of the secondmedical image dataset. In this case, image components of the firstmedical image dataset can be subtracted from image components of thesecond medical image dataset, or vice versa. If the global image datasubtraction is performed before the symmetry subtraction, then the firstmedical image dataset and the second medical image dataset areincorporated unchanged into the global image data subtraction. If theglobal image data subtraction is performed after the symmetrysubtraction, image components of the first medical image dataset and thesecond medical image dataset that changed in the symmetry subtractionare incorporated as input parameters in the global image datasubtraction.

It may be beneficial in this regard to align the image components of thefirst medical image dataset and the second medical image dataset to oneanother, for example by a registration, for the purpose of the globalimage data subtraction. This enables movements of the examinationsubject that have occurred between the recording of the first medicalimage dataset and the second medical image dataset to be at leastpartially compensated for. It is also possible for more than two medicalimage datasets of the examination subject to be acquired. In that case,a number of global image data subtractions can be performed. It is alsopossible for a number of medical image datasets to be merged into anaveraged medical image dataset for the subsequent post-processingoperation. Other processing steps deemed beneficial by those skilled inthe art can also be performed during the global image data subtraction.

The symmetry subtraction takes place within a single medical imagedataset. If the symmetry subtraction is performed before the globalimage data subtraction, two symmetry subtractions will advantageously beperformed separately in each case in the first medical image dataset andin the second medical image dataset. If the symmetry subtraction isperformed after the global image data subtraction, the symmetrysubtraction will advantageously be performed within a result imagedataset of the global image data subtraction, referred to as a globalimage data subtraction image dataset.

In the symmetry subtraction, the first side of the organ system issubtracted from the second side of the organ system, or vice versa.Thus, halves of the organ system that are bilaterally symmetrical to oneanother are subtracted from one another in the symmetry subtraction. Inthe case of brain imaging, for example, one cerebral hemisphere of theexamination subject is subtracted from the other cerebral hemisphere inthe symmetry subtraction. The symmetry subtraction advantageouslyenables laterally different diagnostic findings to be highlighted, sincelaterally identical diagnostic findings typically cancel one another outdue to the symmetry subtraction in the medical image dataset.

For the symmetry subtraction, it may be necessary in the first instanceto determine a plane of symmetry of the organ system. This plane ofsymmetry is determined in particular in such a way that the mirroring ofthe first side onto the second side of the organ system is accomplishedwith a maximum degree of congruence. The first side and the second sideof the organ system can then be localized on opposite sides of the planeof symmetry. In the typical case the aforementioned median plane of theexamination subject will form the plane of symmetry. The symmetrysubtraction can then be a mirroring of the first side of the organsystem onto the second side of the organ system at the plane ofsymmetry, or vice versa. Furthermore, the symmetry subtraction caninvolve a registration, such as a non-rigid registration, of the firstside and the second side of the organ system, as described in moredetail below. Further processing steps deemed beneficial by thoseskilled in the art may also be performed during the symmetrysubtraction.

The combination of the global image data subtraction and the symmetrysubtraction proposed according to the invention affords a particularlyadvantageous further processing of the first medical image dataset andthe second medical image dataset. The result image dataset that is thusobtained can contain particularly significant diagnostic information forthose skilled in the particular field. For example, non-specificvariations between the first medical image dataset and the secondmedical image dataset can be distinguished particularly easily fromspecific variations in the result image dataset.

A basic reason for this is that, according to the inventive approach, aphysiological change and/or a change in an image background between thefirst medical image dataset and the second medical image dataset can besuppressed in the result image dataset on account of the symmetrysubtraction. Performing the symmetry subtraction in addition to theglobal image data subtraction can thus ensure that pathological, inparticular one-sided (unilateral), changes between the first medicalimage dataset and the second medical image dataset stand outparticularly clearly in the result image dataset. For example, in thecase of a take-up of contrast agent between the recording of the firstand the second medical image dataset, an unwanted representation of thecontrast agent uptake in healthy tissue can be suppressed in the resultimage dataset owing to the symmetry subtraction.

The inventive method furthermore offers the advantage that it issuitable for automatic execution, without interaction by a user. Thus,the inventive method can be carried out without prior knowledge aboutthe localization of lesions. A manual segmentation of lesions for thepurpose of the comparison between the first medical image dataset andthe second medical image dataset can advantageously be dispensed with.

In an embodiment, the global image data subtraction is performed in afirst step in which a global image data subtraction image dataset isgenerated, while the symmetry subtraction is performed in a second stepwithin the global image data subtraction image dataset. In this way asubtraction of the first medical image dataset and the second medicalimage dataset with respect to time can be performed first, and then thesymmetry subtraction can be performed in a single subtraction imagedataset. This sequence of the global image data subtraction and thesymmetry subtraction offers the advantage that only one symmetrysubtraction needs to be performed. It is, however, also possible, asdescribed below, to perform parts of the symmetry subtraction, forexample a registration of image components and/or a specification of aplane of symmetry, before the global image data subtraction. Theapproach whereby preparations for the symmetry subtraction are madeprior to the global image data subtraction and the actual symmetrysubtraction is performed after the global image data subtraction offersthe advantage that image information in the first medical image datasetand second medical image dataset that will be potentially be lost in theglobal image data subtraction can be used for the preparations for thesymmetry subtraction.

In another embodiment, the symmetry subtraction involves a firstsymmetry subtraction and a second symmetry subtraction, wherein thefirst symmetry subtraction is performed within the first medical imagedataset and the second symmetry subtraction is performed within thesecond medical image dataset in a first step, wherein a first symmetrysubtraction image dataset and a second symmetry subtraction imagedataset are generated, wherein the global image data subtraction isperformed in a second step between the first symmetry subtraction imagedataset and the second symmetry subtraction image dataset. In thisapproach the symmetry subtraction precedes the subtraction with respectto time. In this way image information in the first medical imagedataset and in the second medical image dataset which will possibly belost in the global image data subtraction can be used to optimum effectin the symmetry subtraction, for example for a registration of the twosides of the organ system.

In another embodiment, prior to the symmetry subtraction, a registrationof the image components of the first side and the second side of theorgan system is carried out, the image components of the first side andsecond side of the organ system registered to one another beingsubtracted from one another in the symmetry subtraction within themedical image dataset. Preferably, a non-rigid (elastic) registration ofthe image components is performed in this case. It is also possible touse a rigid registration or another registration technique known tothose skilled in the art. The registration of the two sides of the organsystem ensures that image components of the first side and the secondside of the organ system, which image the same anatomical structures ofthe organ system, are subtracted from one another in the symmetrysubtraction. Particularly advantageously, the first side and the secondside of the organ system can be made congruent and/or mirror-symmetricalby the registration. Prior to the registration it can be beneficial inthis case to mirror either the first side or the second side of theorgan system, a mirrored side of the organ system being generated in theprocess. The registration can then be performed between the mirroredside and the side of the organ system disposed opposite the mirroredside.

In another embodiment, the registration takes place on the basis of thefirst medical image dataset and/or the second medical image dataset,with registration information being generated from the registration, andthe image components of the first side and the second side of the organsystem are registered with one another within the global image datasubtraction image dataset using the registration information. Theregistration information can include a deformation matrix that resultsfrom the registration of the two sides of the organ system within thefirst medical image dataset and/or the second medical image dataset. Thedeformation matrix generated previously on the basis of the firstmedical image dataset and/or second medical image dataset can then beused for the symmetry subtraction within the global image datasubtraction image dataset. In this case the deformation matrix can beapplied to one side of the organ system in the global image datasubtraction image dataset prior to the symmetry subtraction. Similarly,a specification of a plane of symmetry for the symmetry subtractionand/or a mirroring of image components of the global image datasubtraction image dataset can also be carried out on the basis of imageinformation of the first medical image dataset and/or second medicalimage dataset. Image information of the first medical image datasetand/or second medical image dataset can be used in this case before theglobal image data subtraction takes place. This approach is based on theconsideration that image information of the first medical image datasetand/or second medical image dataset will potentially be lost in theglobal image data subtraction. This is because there may be too fewstructures recognizable in the global image data subtraction imagedataset to enable a registration of the two sides of the organ system,or a registration that is sufficiently precise, using only imageinformation of the global image data subtraction image dataset.

In another embodiment, the provision of the result image dataset isdisplaying the first side and/or the second side of the organ system,with subtraction values resulting from the global image data subtractionand the symmetry subtraction being represented by color coding, and withonly one side of the organ system being displayed with color-codedsubtraction values. It is also possible to display both sides of theorgan system. In that case, positive subtraction values can be displayedin a first displayed side of the organ system, and negative subtractionvalues in a second displayed side of the organ system. Informationrelating to a laterality of a lesion can be inferred from themathematical sign of a displayed subtraction value. The display of thefirst side and/or the second side of the organ system can be performedon the basis of information, in particular a user input, that indicateswhich side of the organ system potentially has a lesion and/or whichsigns of the subtraction values are conceivable for a lesion. If it isknown, for example, that a lesion can lead to a diminution of signalintensities (in a perfusion study, for example), then the lesion can bevisualized on the corresponding side of the two displayed sides of theorgan system. The proposed possibilities for displaying the first sideand/or the second side of the organ system can provide an observerskilled in the particular field, for example, with significantinformation about a change in image components between the recording ofthe first medical image dataset and the second medical image dataset.

In another embodiment, the acquisition of the first medical imagedataset involves recording the first medical image dataset at a firstpoint in time by operation of a medical imaging scanner, and theacquisition of the second medical image dataset involves recording thesecond medical image dataset at a second point in time by operation of amedical imaging scanner, and between the first point in time and thesecond point in time, a relevant event occurs that leads to a change insignal values between the first medical image dataset and the secondmedical image dataset. Such a relevant event may be, for example, astimulus such as an optical or acoustic stimulus, to the examinationsubject, an administration of contrast agent, or some otherintervention.

The evaluation computer according to the invention has an input or portto receive a first medical image dataset and a second medical imagedataset, and a processor that has a first subtraction unit and a secondsubtraction unit configured to perform processing of the first andsecond datasets as described above. An output unit provides theprocessing result as a data file for viewing.

In this way the evaluation computer is embodied for performing a methodfor evaluating medical image data that represent an organ system of anexamination subject, the organ system having a first side and a secondside which are characteristically bilaterally symmetrical to oneanother. The first medical image dataset of the organ system of theexamination subject and the second medical image dataset of the organsystem of the examination subject are provided to the processor, thatprocesses the first medical image dataset and the second medical imagedataset to obtain a result image dataset. The first subtraction unit isconfigured to perform a global image data subtraction in which imagecomponents of the first medical image data and the second medical imagedata are subtracted from one another. The second subtraction unit isconfigured to perform a symmetry subtraction in which image componentsof the first side and the second side of the organ system are subtractedfrom one another within a medical image dataset. The output unit isconfigured to provide the result image dataset as an output.

According to an embodiment of the evaluation computer, the firstsubtraction unit and the second subtraction unit are configured toimplement the global image data subtraction in a first step, a globalimage data subtraction image dataset being thereby generated, and toimplement the symmetry subtraction in a second step within the globalimage data subtraction image dataset.

According to another embodiment of the evaluation computer, the firstsubtraction unit and the second subtraction unit are configured toperform the symmetry subtraction as a first symmetry subtraction and asecond symmetry subtraction, the first symmetry subtraction beingcarried out within the first medical image dataset and the secondsymmetry subtraction being carried out within the second medical imagedataset in a first step, a first symmetry subtraction image dataset anda second symmetry subtraction image dataset thereby being generated. Theglobal image data subtraction takes place in a second step between thefirst symmetry subtraction image dataset and the second symmetrysubtraction image dataset.

According to another embodiment of the evaluation computer, the secondsubtraction process is configured to perform, prior to the symmetrysubtraction, a registration of the image components of the first sideand the second side of the organ system, wherein the image components ofthe first side and the second side of the organ system that areregistered to one another are subtracted from one another in thesymmetry subtraction within the medical image dataset.

According to another embodiment of the evaluation computer, the firstsubtraction processor and the second subtraction processor areconfigured to implement the registration on the basis of the firstmedical image dataset and/or the second medical image dataset, withregistration information being generated from the registration, and theimage components of the first side and the second side of the organsystem within the global image data subtraction image dataset beingregistered with one another using the registration information.

According to another embodiment of the evaluation computer, the outputunit is configured to provide the result image dataset by displaying thefirst side and/or the second side of the organ system, with subtractionvalues resulting from the global image data subtraction and the symmetrysubtraction being represented by color coding.

According to another embodiment of the evaluation computer, the firstmedical image dataset is a recording at a first point in time byoperation of a medical imaging scanner and the second medical imagedataset is a recording at a second point in time by operation of themedical imaging device, and between the first point in time and thesecond point in time, a relevant event occurs, which leads to a changein signal values between the first medical image dataset and the secondmedical image dataset.

The medical imaging apparatus according to the invention has anevaluation computer according to the invention. The evaluation computeris configured to send control signals to the medical imaging scannerand/or to receive and/or process control signals in order to carry outthe method according to the invention. The evaluation computer can beintegrated into the medical imaging apparatus. The evaluation computercan also be installed separately from the medical imaging scanner. Theevaluation computer can be connected to the medical imaging scanner. Theacquisition of the first medical image dataset is a recording of thefirst medical image dataset by operation of a scanner of the medicalimaging apparatus. The acquisition of the second medical image datasetalso is a recording of the second medical image dataset by operation ofthe scanner of the medical imaging apparatus. The first medical imagedataset and the second medical image dataset are then transferred to theevaluation computer for further processing. The evaluation computer thusacquires the first medical image dataset and the second medical imagedataset by controlling operation of the scanner.

The invention also encompasses a non-transitory, computer-readable datastorage medium that can be loaded directly into a memory of aprogrammable evaluation computer and program code that cause thecomputer to implement the method according to the invention when theprogram code is executed in the evaluation computer. As a result themethod according to the invention can be executed quickly and in anidentically reproducible and robust manner. The computer must fulfillcertain requirements, such as having a suitable random access memory, asuitable graphics card or a suitable logic unit, so that the methodsteps can be performed efficiently. Examples of electronically readabledata media are a DVD, a magnetic tape or a USB stick on whichelectronically readable control information, in particular software, isstored.

The advantages of the evaluation computer according to the invention,the medical imaging apparatus according to the invention and the storagemedium according to the invention substantially correspond to theadvantages of the method according to the invention, which have beenexplained in detail above. Features, advantages and alternativeembodiment variants cited in connection with the method are applicableto the other aspects of the invention. The functional features of themethod are embodied by corresponding device-related modules, inparticular by hardware modules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a medical imaging apparatus accordingto the invention having an evaluation computer according to theinvention.

FIG. 2 is a flowchart of a first embodiment of the method according tothe invention.

FIG. 3 is a flowchart of a second embodiment of the method according tothe invention.

FIG. 4 is a flowchart of a third embodiment of the method according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a medical imaging apparatus according to the inventioncomprising an evaluation computer 33 according to the invention in ablock diagram.

The medical imaging apparatus can be, for example, a magnetic resonanceapparatus, a single-photon emission tomography (SPECT) apparatus, apositron emission tomography (PET) apparatus, a computed tomography(CT), an ultrasound device, an X-ray device or a C-arm device. Combinedmedical imaging apparatuses formed by any desired combination of anumber of the cited imaging modalities are also possible. In the caseshown the medical imaging apparatus is embodied as an example as amagnetic resonance apparatus 11.

The magnetic resonance apparatus 11 has a detector unit formed by ascanner 13 that has a basic field magnet 17 for generating a strong andconstant basic magnetic field 18. The magnetic resonance scanner 13 hasa cylinder-shaped patient receiving zone 14 for accommodating anexamination subject 15, in the present case a patient, the patientreceiving zone 14 is cylindrically enclosed by the scanner 13 in acircumferential direction. The patient 15 can be introduced into thepatient receiving zone 14 by a patient support 16 of the scanner 13. Forthis purpose the patient support 16 has a patient support table that ismovable inside the scanner. The scanner 13 is shielded externally by ahousing shell 31.

The scanner 13 additionally has a gradient coil arrangement 19 forgenerating magnetic field gradients that are used for spatial encodingduring imaging. The gradient coil arrangement 19 is actuated by agradient control processor 28. The scanner 13 furthermore has aradiofrequency antenna unit 20, which in the case shown is a body coilpermanently integrated into the scanner 13, and a radiofrequency antennacontrol processor 29. The radiofrequency antenna unit 20 is actuated bythe radiofrequency antenna control unit 29 and radiates radiofrequencymagnetic resonance sequences into an examination chamber that issubstantially formed by the patient receiving zone 14, to cause nuclearspins in the patient 15 to deviate from the polarization that isestablished in the main magnetic field 18 generated by the main magnet17. The radiofrequency antenna unit 20 is furthermore designed toreceive magnetic resonance signals from the patient 15 that result whenthe nuclear spins relax from the excitation produced by theradiofrequency sequence.

The magnetic resonance apparatus 11 has a computer 24 for controllingthe basic field magnet 17, the gradient control processor 28 and theradiofrequency antenna control processor 29. The computer unit 24 isresponsible for the centralized control of the magnetic resonanceapparatus 11, such as performing a predetermined imaging gradient echosequence. Control information such as imaging parameters, as well asreconstructed magnetic resonance images, can be provided for a user on apresentation unit, in the present case a display monitor 25, of themagnetic resonance apparatus 11. The magnetic resonance apparatus 11additionally has an input interface 26 via which information and/orparameters can be entered by a user during a measurement procedure. Thecomputer 24 can include the gradient control processor 28 and/or theradiofrequency antenna control processor 29 and/or the display monitor25 and/or the input interface 26.

The illustrated magnetic resonance apparatus 11 can of course havefurther components that are ordinarily present in magnetic resonanceapparatuses. The basic operation of a magnetic resonance apparatus isknown to those skilled in the art, so a detailed description of thefurther components is not necessary herein.

The depicted evaluation computer 33 has a first image data acquisitioninput 34, a second image data acquisition input 35, an output unit 36,and a processor 37 that includes a first subtraction processor 38 and asecond subtraction processor 39. The first and second image dataacquisition inputs may be formed by the same (i.e., a common) inputport.

The scanner 13, serving as an image data acquisition, unit is designedfor recording a first medical image dataset and a second medical imagedataset. The first image data acquisition input 34 and the second imagedata acquisition input 35 of the evaluation unit 33 then receive thefirst medical image dataset and the second medical image dataset fromthe computer 24 of the magnetic resonance apparatus 11. For this purposethe first image data acquisition input 34 and the second image dataacquisition input 35 are connected to the computer unit 24 of themagnetic resonance apparatus 11 in order to enable an exchange of data.In this configuration the output unit 36 is connected to the displaymonitor 25 of the magnetic resonance apparatus 11 so that a result imagedataset determined by the evaluation computer 33 can be displayed. Themagnetic resonance apparatus 11 is therefore configured together withthe evaluation computer 33 for the purpose of performing the methodaccording to the invention for evaluating medical image data.

Alternatively to the display, the evaluation computer 33 can beconfigured in isolation for the purpose of performing the methodaccording to the invention for evaluating medical image data. To thatend, the first image data acquisition input 34 and the second image dataacquisition input 35 of the evaluation computer 33 will load image datafrom a database and/or retrieve image data from a connected medicalimaging apparatus.

FIG. 2 shows a flowchart of a first embodiment of the method accordingto the invention for evaluating medical image data. The medical imagedata in this case image an organ system of an examination subject 15,the organ system having a first side and a second side which arecharacteristically bilaterally symmetrical to one another.

In a first method step 40, a first medical image dataset of the organsystem of the examination subject 15 is received by the first image dataacquisition input 34.

In a further method step 41, a second medical image dataset of the organsystem of the examination subject is received by the second image dataacquisition input 35.

In a further method step 42, the first medical image dataset and thesecond medical image dataset are processed by the processor 37, so as togenerate a result image dataset. The further method step 42 in this casehas a sub-step 42T in which a global image data subtraction is performedby the first subtraction processor 38, in which image components of thefirst medical image data and the second medical image data aresubtracted from one another. The further method step 42 includes asub-step 42S in which a symmetry subtraction is performed by the secondsubtraction processor 39, in which image components of the first sideand the second side of the organ system are subtracted from one anotherwithin a medical image dataset.

In a further method step 43, the result image dataset is provided inelectronic form by the output unit 36.

FIG. 3 shows a flowchart of a second embodiment of a method according tothe invention for evaluating medical image data.

The following description is limited essentially to the differencescompared to the exemplary embodiment in FIG. 2, reference being made tothe description of the exemplary embodiment in FIG. 2 with regard tomethod steps that remain the same. Method steps that remainsubstantially the same are labeled with the same reference numerals.

The embodiment variant of the method according to the invention shown inFIG. 3 includes the method steps 40, 41, 42, 42T, 42S, 43 of the firstembodiment of the inventive method according to FIG. 2. The embodimentvariant of the method according to the invention shown in FIG. 3 hasadditional method steps and sub-steps. An alternative method executionsequence to FIG. 3, which includes only some of the additional methodsteps and/or sub-steps represented in FIG. 2, is also conceivable. Analternative method execution sequence to FIG. 3 can also includeadditional method steps and/or sub-steps.

In a further method step 44, a relevant event takes place intermediatelyin time between the recording of the first medical image dataset at afirst point in time in the further method step 40 and the recording ofthe second medical image dataset at a second point in time in thefurther method step 41. This relevant event, such as an administrationof contrast agent, leads to a change in signal values between the firstmedical image dataset and the second medical image dataset. The firstmedical image dataset accordingly mirrors a state of the examinationsubject 15 prior to the relevant event and the second medical imagedataset mirrors a state of the examination subject 15 after the relevantevent.

In the case shown, the global image data subtraction is performed in thefurther method step 42 in sub-step 42T prior to the symmetry subtractionin sub-step 42S. During the processing of the first medical imagedataset and the second medical image dataset in the further method step42, the global image data subtraction is performed in a sub-step 42T ofthe further method step 42, a global image data subtraction imagedataset being generated in the process. The symmetry subtraction thentakes place in a further sub-step 42S within the global image datasubtraction image dataset.

Prior to the symmetry subtraction, a registration of the imagecomponents of the first side and the second side of the organ systemtakes place in a further sub-step 42R. In the symmetry subtraction, theimage components of the first side and second side of the organ systemregistered to one another can then be subtracted from one another withinthe subtraction image dataset.

In the case shown, the registration is accomplished in a furthersub-step 42P on the basis of the first medical image dataset and/or thesecond medical image dataset. With the use of the registration,registration information is generated, the image components of the firstside and the second side of the organ system within the global imagedata subtraction image dataset being registered onto one another usingthe registration information in the further sub-step 42R.

Furthermore, a symmetry axis can be specified and/or one side of theorgan system can be mirrored onto the opposite side in the furthersub-step 42P on the basis of the first medical image dataset and/or thesecond medical image dataset. The specified symmetry axis and/orinformation from the mirroring can then be registered during theregistration on the basis of the first medical image dataset and/or thesecond medical image dataset in the further sub-step 42P.

In the further method step 43, providing the result image dataset as anelectronic output, the first side and/or the second side of the organsystem are/is then displayed in a sub-step 43D, subtraction valuesresulting from the global image data subtraction in sub-step 42T and thesymmetry subtraction in sub-step 42R being represented by color coding.The result is accordingly visualized in particular in a false colorrendering.

FIG. 4 shows a flowchart of a third embodiment of the method accordingto the invention for evaluating medical image data.

The following description is limited essentially to the differencescompared to the exemplary embodiment in FIG. 2, reference being made tothe description of the exemplary embodiment in FIG. 2 with regard tomethod steps that remain the same. Method steps that remainsubstantially the same are labeled with the same reference numerals.

The embodiment of the method according to the invention shown in FIG. 4includes the method steps 40, 41, 42, 42T, 42S, 43 of the firstembodiment of the method according to the invention as shown in FIG. 2.The embodiment variant of the method according to the invention shown inFIG. 4 includes additional method steps and sub-steps. An alternativemethod execution sequence to FIG. 4, which includes only some of theadditional method steps and/or sub-steps represented in FIG. 2, is alsoconceivable. An alternative method execution sequence to FIG. 4 can alsoinclude additional method steps and/or sub-steps.

In the case shown, the symmetry subtraction is performed in the furthermethod step 42 in sub-step 42S prior to the global image datasubtraction in sub-step 42T. Thus, the symmetry subtraction comprises afirst symmetry subtraction in a sub-step 42S1 and a second symmetrysubtraction in a sub-step 42S2, the first symmetry subtraction beingperformed within the first medical image dataset and the second symmetrysubtraction being performed within the second medical image dataset.

During this process, a first symmetry subtraction image dataset and asecond symmetry subtraction image dataset are generated. The globalimage data subtraction is then performed between the first symmetrysubtraction image dataset and the second symmetry subtraction imagedataset.

Additional steps from the embodiment according to FIG. 3, for example aregistration of the sides of the organ system, can be performed in thiscase as well.

The symmetry subtraction comprises a first symmetry subtraction and asecond symmetry subtraction, the first symmetry subtraction beingperformed within the first medical image dataset and the second symmetrysubtraction being performed within the second medical image dataset in afirst step, a first symmetry subtraction image dataset and a secondsymmetry subtraction image dataset being generated in the process, theglobal image data subtraction being performed in a second step betweenthe first symmetry subtraction image dataset and the second symmetrysubtraction image dataset.

The method steps of the method according to the invention shown in FIGS.2-4 are performed by the evaluation computer 33. For this purpose, theevaluation computer 33 has the necessary software and/or computerprograms, which are stored in a memory unit of the evaluation computer33. The software and/or computer programs comprise program means whichare configured to perform the method according to the invention when thecomputer program and/or the software are/is executed in the evaluationcomputer 33 by means of a processor unit of the evaluation computer 33.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventor to embody within the patentwarranted hereon all changes and modifications as reasonably andproperly come within the scope of his contribution to the art.

I claim as my invention:
 1. A method for evaluating medical image datacomprising: providing a first medical image dataset of an organ systemof an examination subject to a computer, said organ system having afirst side and a second side that are characteristically bilaterallysymmetrical with respect to each other, said first medical image datasetcomprising first medical image data comprising first image components;providing said computer with a second medical image dataset of saidorgan system, said second medical image dataset comprising secondmedical image data comprising second image components; in said computer,processing said first medical image dataset and said second medicalimage dataset in a processing algorithm to obtain a result image by, insaid processing algorithm, executing a global image data subtractionwherein said first image components of said first medical image data andsaid second image components of said second medical image data aresubtracted from one another in order to obtain a global image datasubtraction image dataset, and thereafter executing a symmetrysubtraction, within said global image subtraction data set, in whichsymmetry subtraction image components of the first side and the secondside of the organ system are subtracted from one another; and makingsaid result image dataset available in electronic form from saidcomputer as a data file.
 2. A method as claimed in claim 1 comprising,before said symmetry subtraction, bringing image components of the firstside and image components of the second side of the organ system, withinsaid global image data subtraction dataset, into registration with eachother.
 3. A method as claimed in claim 1 wherein said symmetrysubtraction comprises a first symmetry subtraction and a second symmetrysubtraction, and executing said first symmetry subtraction within saidfirst medical image dataset to obtain a first symmetry subtraction imagedataset and executing said second symmetry subtraction within saidsecond medical image dataset to obtain a second symmetry subtractionimage dataset, and executing said global image data subtraction betweensaid first symmetry subtraction image dataset and said second symmetrysubtraction image dataset.
 4. A method as claimed in claim 3 comprising,prior to said symmetry subtraction, bringing said image components ofthe first side and the image components of the second side of the organsystem into registration with each other within each of said firstmedical image dataset and said second medical image dataset.
 5. A methodas claimed in claim 1 comprising providing said data file of said resultimage dataset to a display monitor in communication with said computerand, at said display monitor, displaying at least one of said first sideand said second side of said organ system with subtraction valuesresulting from said global image data subtraction and said symmetrysubtraction being represented by color coding.
 6. A method as claimed inclaim 1 comprising providing said computer with said second medicalimage dataset that was acquired with a medical imaging apparatus at apoint in time after acquisition of said first medical image dataset withsaid medical imaging apparatus, and wherein an event has occurred insaid examination subject between the respective times of acquisition ofsaid first and second medical image datasets, and comprising executingsaid processing algorithm in said computer to obtain said result imagedataset with a representation of said event in said result imagedataset.
 7. An evaluation computer comprising: an input configured toreceive a first medical image dataset of an organ system of anexamination subject to a computer, said organ system having a first sideand a second side that are characteristically bilaterally symmetricalwith respect to each other, said first medical image dataset comprisingfirst medical image data comprising first image components, and toreceive a second medical image dataset of said organ system, said secondmedical image dataset comprising second medical image data comprisingsecond image components; said computer being configured to process saidfirst medical image dataset and said second medical image dataset in aprocessing algorithm to obtain a result image by, in said processingalgorithm, executing a global image data subtraction wherein said firstimage components of said first medical image data and said second imagecomponents of said second medical image data are subtracted from oneanother in order to obtain a global image data subtraction imagedataset, and thereafter executing a symmetry subtraction, within saidglobal image subtraction data set, in which symmetry subtraction imagecomponents of the first side and the second side of the organ system aresubtracted from one another; and said computer being configured to makesaid result image dataset available in electronic form from saidcomputer as a data file.
 8. A medical imaging apparatus comprising: amedical imaging scanner; a control computer configured to operate themedical imaging scanner to obtain a first medical image dataset and asecond medical image dataset of an organ system of an examinationsubject to a computer, said organ system having a first side and asecond side that are characteristically bilaterally symmetrical withrespect to each other, said first medical image dataset comprising firstmedical image data comprising first image components, and said secondmedical image dataset comprising second medical image data comprisingsecond image components; said computer being configured to process saidfirst medical image dataset and said second medical image dataset in aprocessing algorithm to obtain a result image by, in said processingalgorithm, executing a global image data subtraction wherein said firstimage components of said first medical image data and said second imagecomponents of said second medical image data are subtracted from oneanother in order to obtain a global image data subtraction imagedataset, and thereafter executing a symmetry subtraction, within saidglobal image subtraction data set, in which symmetry subtraction imagecomponents of the first side and the second side of the organ system aresubtracted from one another, said first medical image dataset, and saidsecond medical image dataset, to obtain said result image dataset; andmake said result image dataset available in electronic form from saidcomputer as a data file.
 9. A non-transitory, computer-readable datastorage medium encoded with programming instructions, said storagemedium being loaded into an evaluation computer of a medical imagingapparatus, and said programming instructions causing said evaluationcomputer to: receive a first medical image dataset of an organ system ofan examination subject to a computer, said organ system having a firstside and a second side that are characteristically bilaterallysymmetrical with respect to each other, said first medical image datasetcomprising first medical image data comprising first image components;receive a second medical image dataset of said organ system, said secondmedical image dataset comprising second medical image data comprisingsecond image components; process said first medical image dataset andsaid second medical image dataset in a processing algorithm to obtain aresult image by, in said processing algorithm, executing a global imagedata subtraction wherein said first image components of said firstmedical image data and said second image components of said secondmedical image data are subtracted from one another in order to obtain aglobal image data subtraction image dataset, and thereafter executing asymmetry subtraction, within said global image subtraction data set, inwhich symmetry subtraction image components of the first side and thesecond side of the organ system are subtracted from one another within asame image dataset, before or after said global image data subtraction;and make said result image dataset available in electronic form fromsaid computer as a data file.